Why no cage bolt in high-end front derailleur?

In looking at some high-end front derailleurs (e.g. Shimano XTR), I noticed that their cages didn't have a bolt closure, but rather what seems to be a rivet. For example, take a look at this one: http://www.rscycle.com/site/image_br...m970_08_lg.jpg
Why is that? Is it just that the derailleur is so wonderful that it would be sacrilegious to open its cage to install/remove a chain, and the chain must be opened instead?

Aside from market-category of the designs, you also have to take in to account the age of the design and cost-of-manufacturing concerns. Sure, low-end designs use rivets to lower costs and increase production-speed. Shimano probably recognized that in their newest designs and use rivets in the XTR. The DeoreLX model you posted is at least 10-years old and it's probably more economical to continue using that design than to start a new one using rivets. But you can bet that when a new DeoreLX model is introduced, it's probably going to have rivets.

Sorry, I'm not familiar with a butterfly spread, but I'm guessing it doesn't refer to my unscrewing an FD bolt causing a Shimano executive's roof to fall in Japan...

Sounds like you're saying that the rivets are used for different reasons on low- and high-end -- low-end because it's cheap and high-end because it's secure. The (now-fading?) use of bolts on mid-range FDs reflects an older era when people were expecting to be able to open their FD's, but we frown on that now.

The (now-fading?) use of bolts on mid-range FDs reflects an older era when people were expecting to be able to open their FD's, but we frown on that now.

In the older era of friction shifting, minor distortion of the fd cage was no big deal since you could adjust it's position anywhere you wanted. Index shifting systems are far less tolearant of fd cage configuration so bending it to pop the chain in and out can have a negative effect on shifting.

Yes.
By removing the screw that holds some FD cage together you can slip out the chain which then allows you to lie the bike down, move most of the chain out past the dropout. The chain can be cleaned in a flat tray using degreaser.
You only spread the cage just enough to slip the chain out or you could possibly distort it as others have said.